This invention relates to systems for measuring changes of mass during a dynamic process, and more particularly to systems and devices for directing the flow of condensable gases so that measurements of mass can be precisely obtained.
Modern precision mass measuring systems, such as electromagnetic balances and similar devices, are capable of such precise measurements that the change in mass of a small sample subject to a decomposition reaction can be precisely monitored as changes occur, thus revealing needed information about the mass that is under examination. This technique is used with modern thermogravimetric instruments that are widely employed in industry and scientific laboratories. In such instruments, a test quantity on a sample holder is held in a furnace and coupled to a precision balance. The sample holder is typically suspended within a thermogravimetric chamber on a thin hang-down wire, the chamber passing through the furnace, which heats the sample to a fixed or changing temperature level. The hang-down wire is coupled at its outer, cool, end to the balance, which is protected in a chamber by a purge gas atmosphere. The sample is usually thermally decomposed under the influence of temperature, with or without a reactive gas, with the dynamic variations in mass being recorded for subsequent analysis. Obviously, what is desired is to obtain a reading which represents changes in mass of the sample only, but this has heretofore been very difficult to achieve because of the difficulty of keeping the flowing gas, after decomposition, away from the cooler portions of the hang-down wire and associated structure. In a typical case, products of decomposition are impelled, along with the purge gas, toward an outlet from the furnace. The gas tends to move upwardly, toward and along the hang-down wire, and a cooler region is reached in which these products condense on the hang-down wire, introducing an increasing error in the mass reading as the reaction proceeds. Condensation may also occur on the chamber walls, rendering them opaque and requiring frequent cleaning or replacement.
Because of the high resolution of the measurements involved, the condensation of matter on the hang-down wire cannot be ignored, but must be compensated for imperfectly by calculation or subsequent independent measurement. It is not feasible to expand the hot zone so as to limit condensation problems because a cooler region must inevitably be encountered and the sample support mechanism and thermogravimetric chamber should not be excessively large. Also, flows and relationships should be kept as invariant as possible. For example, the gas composition in the region of the sample can vary if the effluent, reactant and purge gases are not maintained in the proper relationship. The reactant also should reach the sample at substantially the same temperature and with the same concentration at all times, which is not the case if the flows vary with time. Mixing of reactant with purge gas at the sample must be eliminated to the fullest extent possible in order to maintain constancy of the gas composition. Also, the measurements being made are so sensitive that the differential pressure exerted by gas flows affect the readings and thus must be held constant,.